battery charges but switches off and comes up with itunes restore

iphone 6 s plus dropped a few times , touch screen went so left it in a drawer for 6 months, now replaced the screen which is working now , BUT now i have a new problem charged the battery all night to 100% while using the phone it turned off after about 5 mins then came up with itunes restore , got this off turned the phone on 1% battery , charged for a min went to 100% , tried using again , again switched off itunes restore , i have restored the iphone to new and from a restore from icloud , and now the home button wont work but siri would randomly jump think that is a different problem anyway , so does anyone know if it is the motherboard or the battery , i have tried with a old battery but still the same